Composite Materials Research Progress

290
S.C. Tjong
NC Al, and the NC Al and the CG Al are alternately distributed. (Fig.17(a)). This bimodal
composite exhibits very high compressive yield strength of 1065 MPa comparing to 504 MPa
of bimodal 5083 alloy. However, the composite still exhibits low compressive ductility
(0.8%). Annealing the composite at 723 K improves its ductility to 2.5%. Fig. 18 shows the
temperature dependence of the yield strength for the tri-modal composite. The yield strength
decreases rapidly with test temperatures up to 473 K, followed by a relatively slow decrease
at higher temperatures. At 473 K, the compressive yield stress of the tri-modal composite is
282 MPa, being higher than that of the heat-treated 5083 alloy at room temperature [15]. In
another study, Scheonung and coworkers fabricated bimodal 5083Al/6.5vol.% SiC (25 nm)
composite by blending cryomilled composite powders with an equal amount of CG 5083 Al
followed by HIP and hot rolling [56]. Such nanocomposite exhibits improved tensile ductility
of 2.6% when compared with the nanocomposite consolidated from 100% of the cryomilled
composite powders having a tensile ductility of 0.5%. This is because the ductile coarsegrains
can undergo larger extent of plastic deformation, while ultrafine grains exhibit limited
deformation.
Figure 17. Bright field TEM images for the bimodal 5083Al/10 wt% B 4C composite in the (a) extrusion
direction and (b) transverse direction, with the inset being the selected area diffraction patterns taken at
the interface between the NC Al and B 4C [15].
The creep behavior of the UFG composites is now considered. Presently, little is known
regarding the high temperature creep behavior and deformation mechanism of such
composites. What is the effect of reinforcing particles on the UFG composites having large
grain boundary areas? Would these particles act as effective obstacles to the dislocation
movement and hinder grain boundary sliding and diffusional flow during high temperature
creep? If they do, the creep rates of UFG composites would reduce dramatically. More
research work is needed in this area in near future to elucidate these problems. Nevertheless,
proper understanding of the creep behavior of near-nanostructured Al-based alloy shed light
on the creep deformation of UFG composites. Very recently, Chauhan et al. investigated the
creep behavior of an UFG Al 5083 alloy at 573 – 648 K [57]. The alloy was prepared by
consolidating cryomilled powders via HIP and extrusion. Analysis of the creep date reveals
the presence of a temperature dependence threshold stress. Incorporation of this threshold